1. Field of the Invention
This invention relates to a liquid crystal display device, which is effective as display means for vehicle gauges using liquid crystal and, more particularly, to an assembly comprising a display section including a liquid crystal element having a liquid crystal cell containing sealed liquid crystal, an electric circuit section including a circuit board provided with electronic components for controlling the display on the display section and leads, a retainer for holding the display section, and a light transmissive carrier consisting of a light transmissive material for illuminating the display section at night.
2. Prior Art
Recently, electoronics is being employed extensively in various fields. Electronics is also used in automobile and motorcycle gauges, that is, the conventional speedmeters and rotation meters are being replaced with gauges in which the rotations of wheel axle per unit time, i.e., the running speed, and ignition pulses per unit time, i.e., the engine rotational speed are displayed on a display section by detecting corresponding pulse signals.
The display devices of these display systems can suitably use a liquid crystal element, in which the transmission of light through it is controlled for display according to changes in the direction of orientation of molecules caused with application of a voltage.
Liquid crystal elements consume less power than display devices of other types such as light-emitting diodes and fluorescent display tubes and can also be made to have a thin, flat shape. From the point of view of weight, furthermore, they are superior to other thin, flat display devices such as plasma display panels. In addition, they can not only have a long lifetime themselves but also reduce the rate of power consumption on the batteries carried on the vehicle. They require less space for mounting and the overall weight of the gauge can be reduced. These are some of the reasons why they are extremely useful as display means for vehicle gauges.
Display devices using liquid crystal elements are disclosed in U.S. Pat. No. 4,012,117, U.S. Pat. No. 4,118,111 and U.S. Pat. No. 4,042,294. In one of the disclosed display devices, a liquid crystal element, which comprises a liquid crystal cell including liquid crystal sealed between a pair of transparent electrode substrate and polarizer applied to the outer surface of the liquid crystal cell, is disposed via a conducting member on a wiring circuit board which provides driving signals for controlling the display on the liquid crystal element according to an electric signal representing the state of a corresponding detection spot. In this device, the liquid crystal element is locked to the wiring circuit board via the conducting member by a retainer member, which is secured to the wiring circuit board such that it is in contact with and covers edge portions of the front side and edges of the liquid crystal element. In another disclosed device, an illumination light source is disposed on the rear side of a liquid crystal element. The liquid crystal display device is placed in a gauge case provided with a front glass, and its wiring circuit board is secured to the gauge case.
The liquid crystal display devices disclosed in the above references, however, contain a large number of parts, so that their assembly requires a great deal of man-hours and they are inferior in operation control. In particular, the conducting member must be positioned with great care to obtain accurate electric connections between the lead terminals of the liquid crystal element and the lead terminals of the wiring circuit board. When securing the retainer to the wiring circuit board, furthermore the conducting member is liable to become displaced from its proper position. In this case, failure of display or erroneous display of the liquid crystal element would result. For this reason, great care is required for the assembly, so that the assembly operation requires great care and long time and it is difficult to improve the operation control of the assembly.
In general, liquid crystal display devices for vehicle gauge include a large number of component parts. Parts such as circuit boards, a reflector, a light transmissive member and liquid crystal elements are sequentially placed one on top of another while flanges on the housing press the liquid crystal elements from above so that these components can be assembled while they are held together. These constituent parts, however, are not designed to maintain any positional relationships among them, nor is there any particular part which may serve as the "foundation piece". When these many component parts are piled up one by one sequentially, therefore, errors in alignment, even if each negligibly small, may add up so that the total misalignment from the bottom to the top component can be quite significant. It takes both attention and time to prevent these alignment errors and this affects the production efficiency. Moreover, since the component parts are held together only by the pressure in the vertical direction, the assembly is weak against externally caused vibrations. Since there is provided no positioning means against vibrations in transverse directions, large displacements are likely to result and this may prove to be a fatal defect if the device is used as a vehicle gauge.
In U.S. Pat. No. 4,367,467, there is disclosed a method of assembling a circuit board, a holder, a liquid crystal display, a lens and a cover member. According to this method, the circuit board serves as the "foundation piece" and tabs on the holder engage in apertures in the circuit board while cutout sections on the circuit board are engaged with the clips on the cover member. The liquid crystal display is placed inside the holder shaped like the frame and held by a spring means, while the lens between the liquid crystal display and the cover member is pressed in place by wing-like structures. Thus, this method is advantageous in that the assembly can be made easily without the need for screws or bolts. This device, however, is not suited for use as a vehicle gauge for the following reasons. Firstly, the frame-like holder can be deformed easily by external vibrations which are unavoidable when the device is used as a vehicle gauge. Secondly, the liquid crystal display held by a spring means is likely is to start vibrating strongly in response to external vibration. Thirdly, a vehicle gauge must be usable at night as well as during daylight hours but there is no means provided for guiding a light beam from the light source to the back side of the liquid crystal display. In short, the device disclosed in this patent includes only a relatively small number of components so that the total misalignment will also be relatively small. Thus, the method of assembly disclosed therin may be satisfactory regarding such devices with relatively few components to be assembled. For assembling a display means usable as a vehicle gauge and comprising a much larger number of components, therfore, a different assembly method must be devised and utilized for producing a device which can be used both during the day and at night and which is vibration resistant.
In a still further aspect, liquid crystal display devices of the so-called transmitting type, which are illuminated from the rear side of the liquid crystal element, may be suited for display when the ambient light intensity is low such as at night, but the display becomes somewhat less clear to the eyes when the ambient light intensity is higher than the light intensity of the illumination source such as during daytime. This may be overcome by using an illumination light source having high illumination capacity. The high illumination capacity light source, however, is bulky, and there is a limitation on the space on the rear side of the liquid crystal element. In addition, the power consumption and heat generation are increased. In paticular, an increase of the heat generation in the illumination light source increases the heating of the liquid crystal element, so that erroneous display or failure of display of the liquid crystal element is likely to result. As for liquid crystal display devices of the so-called reflecting type having a reflector on the rear side of the liquid crystal element instead of an illuminating light source, on the other hand, the ambient light may be utilized for display during the day but there may usually be not enough light available at night for display. In order to overcome this problem, it again may be necessary to provide a light source in front of the liquid crystal element.
In a still further aspect, if a tungsten lamp is used because it is readily available and its running cost is comparatively low, the liquid crystal element cannot be illuminated with a uniform intensity since the lamp offers only a point light source. In other words, the brightness varies, with the light intensity being higher for portions closer to the light source and lower for portions more remote from the light source. This is undesirable from the standpoint of easy and accurate reading of the display on the liquid crystal element.